An electric power steering apparatus that energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. And then, in order to supply a current to the motor so that the motor generates a desired torque, an inverter is used in a motor drive circuit.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque T detected by the torque sensor 10 and a velocity V detected by a velocity sensor 12, and controls a current supplied to the motor 20 based on a voltage command value E obtained by performing compensation and so on with respect to the current command value in a current control section. Furthermore, it is also possible to receive the velocity V from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T detected by the torque sensor 10 and the velocity V detected by the velocity sensor 12 are inputted into a current command value calculating section 101. The current command value calculating section 101 decides a current command value Iref1 that is the desired value of the current supplied to the motor 20 based on the steering torque T and the velocity V and by means of an assist map and so on. The current command value Iref1 is added in an addition section 102A and then the added value is inputted into a current limiting section 103 as a current command value Iref2. A current command value Iref3 that is limited the maximum current, is inputted into a subtraction section 102B, and a deviation Iref4 (=Iref3−Im) between the current command value Iref3 and a motor current value Im that is fed back, is calculated. The deviation Iref4 is inputted into a PI control section 104 serving as the current control section. The voltage command value E that characteristic improvement is performed in the PI control section 104, is inputed into a PWM control section 105. Furthermore, the motor 20 is PWM-driven through an inverter 106 serving as a drive section. The current value Im of the motor 20 is detected by a motor current detector 107 and is fed back to the subtraction section 102B. In general, the inverter 106 uses FETs as switching elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensation section 110 is added in the addition section 102A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation section 110 adds a self-aligning torque (SAT) 113 and an inertia 112 in an addition section 114, further adds the result of addition performed in the addition section 114 and a convergence 111 in an addition section 115, and then outputs the result of addition performed in the addition section 115 as the compensation signal CM.
In the case that the motor 20 is a 3-phase brushless motor, details of the PWM control section 105 and the inverter 106 become a configuration such as shown in FIG. 3. That is, the PWM control section 105 comprises a duty calculating section 105A that calculates PWM duty command values D1˜D6 of three phases according to a given expression based on the voltage command value E, dead time sections 105C1˜105C3 that set a dead time with respect to the PWM duty command values D4˜D6 respectively, and a gate driving section 105B that drives each gate of FET1˜FET3 by the PWM duty command values D1˜D3 and simultaneously switches on/off after driving each gate of FET4˜FET6 by PWM duty command values D4d˜D6d that the dead time from the dead time sections 105C1˜105C3 is set respectively. The inverter 106 comprises a three-phase bridge having top and bottom arms comprised of FET1 and FET4, top and bottom arms comprised of FET2 and FET5, and top and bottom arms comprised of FET3 and FET6, and drives the motor 20 by being switched ON/OFF based on the PWM duty command values D1˜D3 and D4d˜D6d. 
Here, the reason for setting the dead times by the dead time sections 105C1˜105C3 is the following.
Every the top and bottom arms that comprise the inverter 106, for example, FET1 and FET4 alternately repeat ON/OFF, in the same way, FET2 and FET5 alternately repeat ON/OFF, and also FET3 and FET6 alternately repeat ON/OFF. However, FET is not an ideal switch and requires a turn on time Ton and a turn off time Toff without instantly performing ON/OFF as instructed by gate signals. As a result, for example, when an ON-instruction for FET1 and an OFF-instruction for FET4 are issued at the same time, FET1 and FET4 become ON at the same time and there is a problem that the top and bottom arms short. Therefore, in order not to generate a flow-through current by turning FET1 and FET4 on at the same time, in the case of giving an OFF-signal to the gate drive section 105B, by giving an ON-signal to the gate drive section 105B after the elapse of a given time called the dead time in the dead time section 105C1 without giving an ON-signal to the gate drive section 105B immediately, short of the top and bottom arms comprised of FET1 and FET4 can be prevented. In the same way, this is applied to other FET2˜FET6 as well.
However, existence of the above dead time becomes a cause that causes problems such as insufficient torque and torque ripple for control of the electric power steering apparatus.
At first, the dead time, the turn on time and the turn off time will be described with reference to FIG. 4. The duty command value D1 (D4) from the duty calculating section 105A shown in FIG. 4(A), is set as an ON/OFF-signal with respect to FET1 and FET4. However actually, a gate signal K1 shown in FIG. 4(B) is given to FET1, and a gate signal K2 shown in FIG. 4(C) is given to FET4. That is, with respect to both of the gate signals K1 and K2, a dead time Td is ensured. A terminal voltage comprised of FET1 and FET4 is set as Van shown in FIG. 4(D). Even the ON-signal based on the gate signal K1 is given, FET1 turns on after the elapse of the turn on time Ton without performing ON immediately. Further, even the OFF-signal is given, FET1 turns off after the elapse of the turn off time Toff without performing OFF immediately. In addition, “Vdc” is a power-supply voltage (a voltage of the battery 13) of the inverter 106. Therefore, a total delay time Ttot is indicated by the following Expression 1.Ttot=Td+Ton−Toff  (Expression 1)
Next, influences on the electric power steering apparatus by the dead time Td will be described.
Firstly, an influence on the voltage is as follows. As shown in FIG. 4, with respect to the ideal gate signals (D1, D4), the actual gate signals K1 and K2 become signals that are different from the ideal gate signals due to the influence of the dead time Td. As a result, although voltage distortion occurs, in the case that the direction of the motor current Im is positive (i.e. in the case that the direction of the current flows from the power supply to the motor), that distortion voltage ΔV becomes the following Expression 2, and in the case that the direction of the motor current Im is negative (i.e. in the case that the direction of the current flows from the motor to the power supply), that distortion voltage ΔV becomes the following Expression 3.−ΔV=−(Ttot/Ts)·(Vdc/2)  (Expression 2)where“Ts” is an inverse number (Ts=1/fs) of a PWM frequency
fs in the case of PWM-controlling the inverter 106.ΔV=(Ttot/Ts)·(Vdc/2)  (Expression 3)
By representing the above Expressions 2 and 3 in one expression, the following Expression 4 can be obtained.ΔV=−sign(Im)·(Ttot/Ts)·(Vdc/2)  (Expression 4)
where sign (Im) represents the polarity of the motor current Im.
It is derived from the above Expression 4 that when the PWM frequency fs is high and the power-supply voltage Vdc is large, as the distortion voltage ΔV is high, the influence of the dead time Td greatly appears.
Although the influence of the dead time Td with respect to the voltage distortion is described as above, even with respect to the current or the torque, there are undesirable influences caused by the dead time Td. With respect to current distortion, when the current changes from positive to negative or from negative to positive, the dead time Td causes a zero clamping phenomenon (i.e. a phenomenon that the current sticks to the vicinity of zero). This is because, since a load (the motor) is inductance, there is a trend that voltage drop caused by the dead time Td keeps the current at zero.
Further, the influence of the dead time Td with respect to the torque, appears in an insufficient output torque and an increase in torque ripple. That is, the current distortion generates a low order harmonics, and that is conducive to the increase in the torque ripple. Moreover, since the actual current that is affected by the dead time Td, becomes smaller than the ideal current, the lack of output torque occurs.
In order to prevent such an undesirable influence of the dead time Td, various measures (so called “dead time compensation”) are considered. The basic concept is to compensate the distortion voltage ΔV shown in the above Expression 4. Therefore, the compensating expression 4 is to correct by means of a dead time correction value (voltage) Δu shown in the following Expression 5.Δu=sign(Im)·(Ttot/Ts)·(Vdc/2)  (Expression 5)
In the dead time compensation, there is a problem that it is impossible to accurately detect the polarity sign(Im) of the current Im. When measuring the polarity of the current Im, noises of the PWM control and the above-described zero clamping phenomenon of the current make it difficult to accurately measure the polarity of the current Im.
Furthermore, in the electric power steering apparatus, in a straight running, with respect to characteristics of the vicinity of a steering neutral position, a fine control such as repeating a steering reverse with a weak current is required constantly. In particular, since it is a straight running state, for example, in running at a high speed, road vibration being transmitted to the steering wheel is small, thus unstable elements of the assist easily transmit as vibrations.